This invention relates to the formation of multi-layer coatings on elongated strip articles. More particularly, the invention relates to the formation of coatings having two or more layers of coating materials on the surfaces of elongated strip articles, e.g. aluminum sheet material.
There are many reasons why it is desirable to coat elongated strip articles, e.g. aluminum sheet, with layers of coating materials. For example, such coatings can provide the underlying strip material with protection against attack by harmful chemicals, the atmosphere or pollution, etc. Moreover, in the food industry, it may be desirable to protect packaged articles (e.g. foodstuffs) from attack by or contamination with components of the material forming the elongated strip articles used for packaging.
While single coating layers may be used for these purposes, multiple coating layers of different materials are frequently advantageous. For example, it may be advantageous to provide an inner layer that has good adhesion to the underlying surface and an outer layer having good lubricity for forming operations, or other desirable characteristics, such as peelability, product release characteristics, etc.
While coating materials used for this purpose are often solids dissolved or suspended in volatile solvents or aqueous media (e.g. conventional paints), in some cases it is more desirable to use molten polymers that are coated directly onto substrate surfaces and allowed to cool and harden, or to use liquid polymers that are subsequently cured by heat or radiation. The use of undissolved polymers has the advantage that atmospheric pollution by solvent vapors can be avoided.
Methods of and apparatus for applying multi-layer coatings of materials onto suitable elongated substrates are already known, as briefly described below.
U.S. Pat. No. 2,761,418, which issued on Sep. 4, 1956 to T. A. Russell, discloses a multiple coating apparatus intended primarily for producing photographic film. The apparatus uses a coating head capable of simultaneously applying two layers to a surface of a moving web of material.
U.S. Pat. No. 3,413,143, which issued on Nov. 26, 1968 to E. Cameron et al., discloses a method of and an apparatus for applying a liquid to a moving web, again primarily intended for coating photographic materials. The apparatus employed a coating head capable of simultaneously applying multiple coatings of different materials.
U.S. Pat. No. 3,573,965, which issued on Apr. 6, 1971 to Mamoru Ishiwata et al., discloses an improved so-called multiple doctor coating method. This involves the use of a coating head having multiple liquid chambers and coating slots leading from the liquid chambers to the coating face. The parts of the coating face between the slots form doctor edges which control the flow of the coating materials onto the moving substrate surface.
U.S. Pat. No. 5,072,688, which issued on Dec. 17, 1991 to Chino et al., describes a process and apparatus for producing multi-layer coatings useful for magnetic recording media. The coatings are produced by an extrusion-type coating head in which different coating solutions are pumped into different pockets formed in the head and are passed through narrow slits meeting at a coating slot formed at the ends of the slits.
U.S. Pat. No. 5,186,754, which issued on Feb. 16, 1993 to Umemura et al., discloses an extruder for coating magnetic layers onto a tape. The coating is produced by a coating head provided with two liquid reservoirs, each having an outlet channel. The channels merge before reaching the coating surface of the coating head to form a single coating slot.
International (PCT) patent publication WO 94/03890 published on Feb. 17, 1994 in the name of BASF Magnetics GmbH discloses a coating arrangement for magnetizable coatings having a coating head provided with a particular geometry and utilizing a magnet to ensure a stabilized coating.
While these known arrangements are capable of producing multi-layer coatings on substrates, they are mostly intended for use with very thin flexible substrates of uniform thickness, such as the backing material ribbon used for magnetic tapes or photographic films. All of the known arrangements employ coating heads held in a fixed position relative to a path normally followed by the substrate to be coated. However, such arrangements are not well suited to the application of thin coatings to metal strip articles, such as aluminum sheet, because they cannot easily adjust to the variations in thickness and surface height characteristic of moving strip articles of this kind. Therefore, they cannot easily be employed for the type of multi-layer coating contemplated above since coating layers having unacceptable variations in thickness are thereby produced and, in some cases, the fixed coating head may contact the surface to be coated, resulting in damage.
There is therefore a need for an improved coating method and apparatus for forming multi-layer coatings on elongated strip articles of the type mentioned above.